Fertility Road Issue 06

SCIENCE | counting chromosomes
The
Science behind
COUNTING
CHROMOSOMES
A number of recent scientific publications have shown improved pregnancy
rates following the use of new technology that counts the number
of chromosomes in embryos prior to implantation in IVF treatments.
Numbers not adding up
Biology tells us that fertilisation combines 22 chromosomes and
an X chromosome from the mother’s egg with 22 chromosomes
and either an X or Y chromosome from the father’s sperm to produce
an embryo with 46 chromosomes which, in the case of a girl,
includes two X chromosomes and, in the case of a boy, includes
one X and one Y chromosome. Unfortunately errors in this process
can happen and result in an incorrect number of chromosomes in
the embryo, a condition termed aneuploidy.
Trisomy 21 Disorder, better known as Down syndrome, is the
most common viable human aneuploidy that increases with the
age of the mother. Many scientists believe that aneuploidy is an
important cause of infertility, but until recently the technical
difficulties of counting chromosomes in a single human cell have
made it very difficult to draw firm scientific conclusions.
In most IVF cycles only one embryo is implanted, choosing the
most potentially viable embryo is of paramount importance. The
selection of a single embryo is typically made using morphological
and scorecard techniques, however the ability to screen embryos
for aneuploidy makes it conceivable that in future, only embryos
with the correct number of chromosomes will be considered
for implantation.
Chromosome counting
The technique that underpins the ability to count chromosomes
is termed microarray hybridisation. This approach enables thousands
of independent measurements to be made across all chromosomes.
Microarrays are not new; they are already in routine
%
80
70
60
50
40
30
20
10
0
Oocyte aneuploidy and maternal age
Aneuploidy (%)
Live Birth (%)
Miscarriage (%)
30 31 32 33 34 35 36 37 38 39 40 41 42 43 44
Maternal age
58 fertility road | may - june